Knowing how to stop the opposition and actually doing it are two very different things.

South Africa have expanded their gameplan since becoming world champions four years ago, but the platform for that attack is still laid by the forwards.

On their day, nobody can put their size, weight and power to better use than the Springboks, and how Ireland deal with that will go a long way to deciding their contest on Saturday.

It's been a largely positive 160 minutes from Ireland so far in France, with Saturday's 59-16 victory over Tonga following the previous trouncing of Romania, while aside from Finlay Bealham, they appear to have come through the opening games without serious injury.

The difference between playing the likes of Romania and Tonga, and taking on South Africa, is incomparable though and it will require Andy Farrell's side to considerably raise their game next Saturday.

There's plenty of familiarity between the sides. Ireland were 19-16 winners when the sides met in Dublin last November, and as Ross Byrne explains, the regular meetings with the South African franchises in the URC has removed some of the mystique around them.

That's only a small part of the battle though.

"We're probably used to playing them in the URC all the time," he said, after Ireland's win against Tonga in Nantes.

"But I think everyone knows what South Africa offer, so we’re all aware of what is going to be coming from them. It’s all about us putting in the best performance we can."

The 28-year-old came off the bench for the final 40 minutes of Saturday's victory, one of four players to have made their World Cup debuts in the second half.

It was a long overdue World Cup bow for Byrne, who missed out on selection four years ago at the expense of Joey Carbery and Jack Carty.

And the out-half admits it's been a long journey to tick off a lifetime ambition.

"It certainly has yeah.

"I'm absolutely delighted, most importantly we put in a good performance second half. Start was a little bit sticky, we were a bit stop-start, we got into our rhythm which was pleasing."

Byrne played 40 minutes off the bench, steering Ireland home with relative ease at Stade de la Beaujoire, kicking four conversions from four attempts and setting up Bundee Aki for the first of his four tries.

"It's something I’ve become used to over the last few years of my career," added Byrne of that bench role.

"I’ve done it plenty of times, so yeah, it’s see what’s happening throughout the game and if there is something I can add, or if there is any different pictures or whatever, and obviously we’re talking in the changing room as well, different messaging and stuff like that, just trying to implement the plan as best I can."

It seems to be a 50/50 call between Byrne and Jack Crowley for the number 22 shirt in Paris this week, with both players having appeared off the bench in the opening two games, and shown good flashes in their respective cameos.

A starting role looks to be out of the question for both though given the form Johnny Sexton is in.

The Ireland captain has scored three tries among 40 points so far in the tournament, and passed out Ronan O'Gara as Ireland's record points scorer with his try on Saturday night.

"He deserves it, he's been absolutely incredible for well over a ten-year period. I’m delighted for him. There is not really much I can say about him.

"He’s got plenty more to go in the next few weeks.

"He's been doing it for so long. The longevity he’s had has been incredibly impressive. Very few, if anyone else, who has done that. It is remarkable."

Watch live coverage of Ireland v South Africa (Saturday, 8pm) on RTÉ2 and RTÉ Player, listen to live commentary on RTÉ Radio 1, and follow live updates on RTÉ Sport Online and the RTÉ News app.